With traditional constant volume samplers (CVS), engine exhaust is diluted with ambient air, and a small sample of the diluted exhaust is proportionally extracted and stored in one or more sample bags. Depending upon the engine size, drive cycle and ambient conditions, the CVS total flow rate, which includes both ambient air and engine exhaust, is selected to ensure the sample collected does not condense water when stored in the bags, or during subsequent analysis. This flow rate is determined by calculating the average dew point in the bag sample.
It is desirable to avoid water condensation within the sample bag for several reasons. First, condensation of water impacts the accuracy of the exhaust analysis. Some substances in the exhaust become soluble in water. These substances can be effectively “pulled out” of the exhaust so that they are not measured at the conclusion of the test. Also, the water vapor that becomes condensed is not measured and included in the test results. Second, the condensation can cause the collection of substances on the inside of the bag as the water subsequently evaporates thereby leaving an undesirable residue that will be present during future tests. Finally, new legislation requires no condensation in the sample bags.
There are several factors that make it difficult to avoid condensation of the sample within the bags. For example, use of alternative fuels, new test cycles and larger displacement engines all can lead to condensation within the sample bags. For example, if an aggressive test cycle is performed and the traditional optimal flow CVS flow rate is selected, then condensation will form. This is particularly true for test cycles where the maximum exhaust comes very early in the collection of the sample. The dew point of the sample may be higher than ambient conditions even though the average water concentration in the bag is less than ambient at the end of the cycle. CVS optimal flow rate is selected to ensure the average water concentration in the bags has a dew point less than ambient temperature.
One potentially problematic test is the newly proposed US06 drive cycle. The cycle is 600 seconds long and the second sample bag used in the test will start filling 133 seconds into the drive cycle. The traditional desired flow rate is 1050 scfm when diluting a gas with a dew point of 18 deg C. For vehicles running on ethanol fuel, the ending dew point in the bag will be just above 23 deg C., with a peak dew point at the beginning of the second bag fill of 27 deg C. This is often higher than ambient conditions in a test cell. In this scenario, the CVS flow rate would typically be selected to dilute for the average bag dew point of 23 deg C., which would result in the sample condensing in the second sample bag due to the initial high peak.
In order to avoid condensation in the bag, the CVS flow rate would have to be raised to 2000 scfm to avoid the initial peak, which is undesirable. Increasing the CVS flow rate would reduce the already low concentration of exhaust within the sample making it more difficult to analyze. One approach that can be used to avoid condensation is to heat the bags, which would maintain the sample gas temperature above the maximum dew point and avoid the initial dew point peak. However, additional equipment must be employed for such an approach leading to a higher cost CVS.
Hybrid vehicles pose unique problems when trying to determine mass emissions rates during emission test sequences. Current test procedures require bag sampling using either a CVS method or a bag mini-diluter (BMD) method. Hybrid vehicles that produce exhaust gas from internal combustion engines may not be in operation or may operate for a brief period of time over the test cycle. When using the CVS method the CVS bag sample is overdiluted and determination of mass emissions is difficult since the dilution factor from the CVS method is very high. When using the BMD method the bag sample is diluted at a fixed rate so the dilution factor issue is resolved but the sample is collected proportional to the exhaust flow. Since there are periods of operation where either no exhaust flow is expelled out of the hybrid vehicle or the vehicle exhaust is expelled intermittently very little exhaust may be emitted during the sample phase. Therefore, very little sample will be collected in the sample bag making accurate analysis more difficult.